Improvement of the LNE’s metrological Atomic Force Microscope …€¦ · Table 1. Parasitic...
Transcript of Improvement of the LNE’s metrological Atomic Force Microscope …€¦ · Table 1. Parasitic...
a Corresponding author: [email protected]
Improvement of the LNE’s metrological Atomic Force Microscope (mAFM) performance: Design of new mAFM head dedicated for nanometrology applications
Younes Boukellal1, a
, Sébastien Ducourtieux1 and Benoit Poyet
1
1Laboratoire National de métrologie et d’Essais, 29, Avenue Roger Hennequin 78190 Trappes, France
Abstract. A metrological Atomic Force Microscope (mAFM) has been developed at LNE [1, 2]. It is mainly used
for performing traceable measurement and calibration of transfer standards dedicated to scanning probe and
scanning electron microscopes. In order to improve the mAFM performance and reduce the measurement
uncertainty, a new mAFM head is being developed and will be integrated on the instrument. It consists of an
immobile AFM head working in a zero detection mode. The head is kinematically mounted on the stationary
part of a home-made piezo-actuated flexure stage that produces three translations with a displacement range of
60 µm along X and Y axes and 15 µm along Z axis. The tip-sample relative position is measured with four dual
pass differential interferometers with an expected uncertainty of about 1 nm. This paper presents the design of
new mAFM and the evaluation of the first uncertainty components of the instrument.
1. The AFM head
Up to now, a commercial AFM head (Park Instruments,
XE-100 AFM head [3]) with a non-optimized metrology
frame is used. This frame is about 10 cm long mainly
composed of aluminium and of course sensitive to
thermal drift. The tip is directly linked to this frame
(figure 1-a). First characterization of the instrument
equipped with this AFM head revealed an unwanted drift
close to 10 nm during a scan of 11 hours on a reference
Figure 1. Topography drift (z direction) measured during the
scan of a reference sample. (a) Identification of the metrology
loop of the commercial AFM head (b) Sample image produced
by recording XYZ positions measured with interferometers. (c)
Topography drift along Z axis.
The drift is mainly due to thermal dilatation of the Z
metrology frame inside the AFM head. The head design
also presents other drawbacks as for example the two
thermal sources contained inside, the one associated to
the laser diode used for the detection of cantilever
deflections and the one produced by the electronic circuit
used for the conditioning of the signal generated by the
quad cell photodiode. Mainly for these reasons, we have
started the development of a new metrological AFM head
with no heat source. The most important requirements of
the design are:
• An immobile AFM head working in a zero detection
mode.
• A resolution of 0.1 nm for the cantilever deflection
measurement with a low noise level below 1nm.
• An optical access to observe and position the tip with
respect to the sample using an optical microscope.
• A tip positioned at the thermal center of the
instrument.
• A tip/sample approach system with no heat
generation.
(a)
(b)
(c)
Metrology
loop
DOI: 10.1051/C© Owned by the authors, published by EDP Sciences, 2013
201/
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This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2 0 , which . permits unrestricted use, distributiand reproduction in any medium, provided the original work is properly cited.
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1.1 The cantilever detection system
The accuracy and the stability for measuring the
cantilever displacement are very important for reaching a
good resolution of mAFM deflexion detection. Many
techniques have been developed for this measurement
based on different physical principles (optical beam,
optical interferometer, tuning fork, capacitive cantilever,
piezo-resistive cantilever…). Our cantilever detection
system is based on the optical beam deflexion method
(OBD) [4], with some specific modifications in order to
eliminate the major heat sources around the tip. The
system and the associated characterization results will be
discussed in poster
Figure 1. The mAFM cantilever deflexion detection system
(Sectional view) with (1) a tip/tilt stage, (2) the super
luminescent diode, (3) a reflecting prism, (4) a reflecting mirror,
(5) the home-made deflexion sensitive detector, (6) the
microscope objective field of view, (7) the cantilever/tip holder.
We decided to use a fibered super luminescent diode
(SLD) (figure 1-(2)) to reduce the coherence length of the
light, thus reducing the optical parasitic signal due to
optical interference. The SLD has a center wavelength of
681.2nm, a spectral width of 9.6 nm and very small
coherence length (28 µm) which is an order of magnitude
smaller than the coherence length of typical laser diode.
The SLD is equipped with a Faraday isolator to attenuate
the back reflected light to the SLD. Temperature is
stabilized by peltier element which is integrated in the
SLD cage. The SLD cage is deported outside the mAFM
room. The light beam from the SLD fiber is reflected by a
prism (figure 1-(3)) and deflected onto the cantilever
(figure 1-(7)). The whole (prism + SLD) are linked to a
tip/tilt stage (figure 1-(1)) to control the position of the
beam on the cantilever and to always maintain the spot
centered on the prism. The spot size on the backside of
the cantilever is equal to 23 µm and the working distance
is 58 mm which frees more space under the SLD to
integrate the OBD components. From the cantilever, the
reflected beam is directed by a mirror (figure 1-(4)) to a
home-made deflexion sensitive detector (figure 1-(5)).
The detector axes are aligned with the cantilever axes. In
order to observe the spot on the cantilever we use a long
working distance microscope objective (figure 1-(6))
(WD = 51 mm, x10, ON 0.21).
1.2 The mirror blocks
Figure 2. The mAFM mirror blocks configuration with (1) the
mAFM head, (2) the reference prism, (3) the cantilever + tip,
(4) a mirror faces, (5) the interferometer beams, (6) the
measurement prism, (7) the Z stage
To measure the relative displacement of the sample with
regards of the tip, the differential interferometers aim at
mirrors (figure 2-(5)) directly machined on two prisms
made of Zerodur® to minimize errors introduced by
thermal expansion. Due to the system geometry, the two
blocks have the shape of a truncated inverted four-face
pyramid with a half top angle of 55◦. The mirrors are
machined and polished to obtain flatness better than λ/10
and an orthogonality deviation of less than 300 µrad
(figure 2-(4)). The first prism (figure2-(2)) is linked to the
AFM head using v-groove kinematic coupling. It
constitutes the reference mirror block for the
interferometers. The tip (figure 2-(3)) is directly linked
to the reference prism which shortens the metrology loop
length. The second one, the measurement mirror block
(figure 2-(6)), is associated with the sample displacement.
It is linked to the Z stage (figure 2-(7)) using a v-groove
kinematic coupling. In this configuration, the whole of
the system is Abbe compliant.
1.3 mAFM head support frame and the tip/sample approach frame
Figure 3. The mAFM head and its support frame with (1) a v-
groove support, (2) the mAFM head support frame, (3) the
piezo-legs motors, (4) The tip sample approach frame.
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Both of the support frame (figure 3-(2)) and the
tip/sample approach frame (figure 3-(4)) are made from
invar. The two structures play an important role in the
thermal design strategy. Indeed the large amount of invar
allows us to improve the thermal inertia and thermal
stability of the setup. Furthermore, the tip/sample
approach frame supports three piezo-legs motors (no heat
generation) (figure 3-(3)) with an integrated linear
encoder. The translations of these motors are first
combined in order to generate the necessary rotations for
the alignment of the reference mirror blocks in regard
with the measurement mirror block. Once the alignment
of the mirrors performed, the motors are synchronized to
generate a vertical translation and thus performed the
tip/sample approach with high resolution steps. The
mAFM head is linked to the motors using three v-groove
kinematic coupling (figure 3-(1)).
A more detailed description of the mAFM head will be
presented on the poster
2. Evaluation of the first uncertainty components
2.1 Evaluation of the translation stage parasitic rotations
The three axis translation stages used on the LNE’s
mAFM has been characterized to determine the impact of
Abbe error in the uncertainty budget. To evaluate in situ
the three parasitic rotations of the translation stage, we
developed a dedicated measurement bench.
Figure 4. measurement bench for parasitic rotation evaluation
It consists of a triple beam plane mirror interferometer
(SIOS, SP-TR Series, with 0.1 nm resolution for length
measurement and 0.01 µrad for angular measurement)
and a mirror holder kinematically mounted on the sample
holder. Because the interferometer is capable of
performing two rotation measurements, the pitch and yaw
rotations have been measured simultaneously in a first
configuration and the roll rotation was measured in a
second one. The parasitic rotations were evaluated while
the stage was controlled in close loop using the mAFM
interferometers. Results are summarized in table 1.
Table 1. Parasitic rotations of the XYZ translation stage
Absolute
rotations
(µrad )
60 µm x
60µm x
15 µm
Tx Ty Tz
Rx RY RZ RX RY RZ RX RY RZ
0.6 6 0.7 5.5 0.5 0.7 4.2 0.7 2.2
Even though the measured rotations are linear and
repeatable (corrections will be possible), they are higher
than the expected 1 µrad and higher than the one
previously measured on a stage prototype [2]. Many
effects has been identified to explain some of the highest
parasitic rotational motions measured for Tx Ry and Ty
Rx– see Table 1: (i) the location of the piezo actuators
with respect to the guidance mechanism, (ii) a lack of
decoupling between the X, Y and Z axis and (iii) a defect
on the flexure stage assembly. It will be presented as well
as the solutions under investigation.
2.2 Evaluation of interferometer measurement
stability
We’ve experimented that the differential interferometer
measurements in ambient air are sensitive to differential
variations of air index between the two arms. Fig. 1
shows that the noise level associated to air index
differential fluctuation is about 20 nm peak-to-peak
without any specific protections. On the other hand,
thanks to an aluminium enclosure along the beam path,
the air index is more homogeneous between the two arms
and more stable over long time measurements. Thus, the
equivalent noise level is reduced with a ratio of 40 to 0.5
nm peak-to-peak (see Figure 4).
Figure 5. (up) interferometer stability measurements, (down)
Allan deviation with the protection.
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We evaluated the noise level the standard deviation when
the beams were protected by using two reference
methods: Allan variance and power spectral density. The
two methods gave the same experimental standard
deviation value, equal to 0.163 nm, which validates the
measurement principle [5]. We also evaluated the long
term position stability over 50 hours with the protection.
The Allan deviation vs. averaging time shows that below
5 seconds, the noise nature is white. Over 5 seconds, the
drift becomes predominant. This experiment has shown
that the beam path enclosure improves the interferometer
measurement stability
2.2 Evaluation of the interferometer non-linearity
The four differential interferometers have nonlinearity
behaviour due to polarisation mixing mainly caused by
misalignment and imperfection of optics used in the
interferometer head. To evaluate its nonlinearities, we
implemented an experimental setup which comprises an
interferometer module from Renishaw (RLD-X3-DI)
with two mirrors. The first one is attached to the XYZ
translation stage (Physik Instrumente P-517C), the
second one is linked to the mAFM tip. A 20 mm thick
aluminium enclosure reduces the effect of air index
fluctuations and the interferometer drift during the
experiment. To minimize the error induced by air
turbulence, the path length of the interferometer is as
short as possible (10 cm). A triangle wave is applied to
the stage controller to provide fine displacement in the
range of several micrometers along the interferometer
axis. The measurements of the stage capacitive sensor are
also used to be compared with interferometer
measurements. As shown on Figure 3, the nonlinearity is
determined by measuring the residual displacement when
the first order of the interferometer versus capacitive
sensor curve is removed.
Figure 6. Interferometer nonlinearity (left) and its associated
distribution (right)
The Lissajous representation (sine vs. cosine signal) gives
information about the quadrature signals quality of
interferometers. For a 100% Lissajous strength, the
nonlinearity is 1.41 nm peak-to-peak. Its periodicity is
consistent with the one expected for a double pass
interferometer (λ/4 ≈ 158 nm). The distribution
associated to the nonlinearity is normal with a standard
deviation of 0.47 nm. In order to evaluate the impact of
the interferometers misalignment, we evaluated the
nonlinearity error variation with respect to the Lissajous
signal strength. The Lissajous signal strength must be
over 50% to perform interferometric measurements with
low nonlinearity error (i.e. 1.41 nm peak to peak). Below
50%, the nonlinearity error becomes significant (i.e. more
than 4 nm peak to peak) and does not meet the expected
uncertainty requirements for the tip-sample relative
position measurement. Thus, it would be useful to find an
optimal alignment before starting measurements on the
mAFM.
Figure 7. Non-linearity variation with respect to the optical
phase shift inside the interferometer head
Conclusion
A new metrological AFM head with no heat sources
around the tip is currently under development in order to
avoid thermal drift of the instruments during scanning.
The 3D design is finished and the next step will be
devoted to the prototype realization.
Experimental investigations gave the first uncertainty
components of the LNE’s mAFM. The nonlinearity error
is not critical in the uncertainty budget. The
interferometer stability evaluation shows the importance
of protecting them to reduce the refractive index
variations between the two arms of each interferometer.
To lower Abbe error to 1 nm, new configurations for the
actuation of flexure stage will be investigated and. All
those experimentally determined uncertainty components
will be used to modelize the mAFM measurement
process and to investigate its measurement uncertainty.
Bibliography
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Saint-Quentin en Yvelines
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Park S 2003 Atomic force microscope with improved scan
accuracy, scan speed, and optical vision Rev. Sci. Instrum. 74
4378–83
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